'\" et
.TH PTHREAD_CLEANUP_POP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
pthread_cleanup_pop,
pthread_cleanup_push
\(em establish cancellation handlers
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
void pthread_cleanup_pop(int \fIexecute\fP);
void pthread_cleanup_push(void (*\fIroutine\fP)(void*), void *\fIarg\fP);
.fi
.SH DESCRIPTION
The
\fIpthread_cleanup_pop\fR()
function shall remove the routine at the top of the calling thread's
cancellation cleanup stack and optionally invoke it (if
.IR execute
is non-zero).
.P
The
\fIpthread_cleanup_push\fR()
function shall push the specified cancellation cleanup handler
.IR routine
onto the calling thread's cancellation cleanup stack. The cancellation
cleanup handler shall be popped from the cancellation cleanup stack and
invoked with the argument
.IR arg
when:
.IP " *" 4
The thread exits (that is, calls
\fIpthread_exit\fR()).
.IP " *" 4
The thread acts upon a cancellation request.
.IP " *" 4
The thread calls
\fIpthread_cleanup_pop\fR()
with a non-zero
.IR execute
argument.
.P
It is unspecified whether
\fIpthread_cleanup_push\fR()
and
\fIpthread_cleanup_pop\fR()
are macros or functions. If a macro definition is suppressed in order
to access an actual function, or a program defines an external
identifier with any of these names, the behavior is undefined.
The application shall ensure that they appear as statements,
and in pairs within the same lexical scope (that is, the
\fIpthread_cleanup_push\fR()
macro may be thought to expand to a token list whose first token is
.BR '{' 
with
\fIpthread_cleanup_pop\fR()
expanding to a token list whose last token is the corresponding
.BR '}' ).
.P
The effect of calling
\fIlongjmp\fR()
or
\fIsiglongjmp\fR()
is undefined if there have been any calls to
\fIpthread_cleanup_push\fR()
or
\fIpthread_cleanup_pop\fR()
made without the matching call since the jump buffer was filled. The
effect of calling
\fIlongjmp\fR()
or
\fIsiglongjmp\fR()
from inside a cancellation cleanup handler is also undefined unless the
jump buffer was also filled in the cancellation cleanup handler.
.P
The effect of the use of
.BR return ,
.BR break ,
.BR continue ,
and
.BR goto
to prematurely leave a code block described by a pair of
\fIpthread_cleanup_push\fR()
and
\fIpthread_cleanup_pop\fR()
functions calls is undefined.
.SH "RETURN VALUE"
The
\fIpthread_cleanup_push\fR()
and
\fIpthread_cleanup_pop\fR()
functions shall not return a value.
.SH ERRORS
No errors are defined.
.P
These functions shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
The following is an example using thread primitives to implement a
cancelable, writers-priority read-write lock:
.sp
.RS 4
.nf

typedef struct {
    pthread_mutex_t lock;
    pthread_cond_t rcond,
        wcond;
    int lock_count; /* < 0 .. Held by writer. */
                    /* > 0 .. Held by lock_count readers. */
                    /* = 0 .. Held by nobody. */
    int waiting_writers; /* Count of waiting writers. */
} rwlock;
.P
void
waiting_reader_cleanup(void *arg)
{
    rwlock *l;
.P
    l = (rwlock *) arg;
    pthread_mutex_unlock(&l->lock);
}
.P
void
lock_for_read(rwlock *l)
{
    pthread_mutex_lock(&l->lock);
    pthread_cleanup_push(waiting_reader_cleanup, l);
    while ((l->lock_count < 0) || (l->waiting_writers != 0))
        pthread_cond_wait(&l->rcond, &l->lock);
    l->lock_count++;
   /*
    * Note the pthread_cleanup_pop executes
    * waiting_reader_cleanup.
    */
    pthread_cleanup_pop(1);
}
.P
void
release_read_lock(rwlock *l)
{
    pthread_mutex_lock(&l->lock);
    if (--l->lock_count == 0)
        pthread_cond_signal(&l->wcond);
    pthread_mutex_unlock(&l->lock);
}
.P
void
waiting_writer_cleanup(void *arg)
{
    rwlock *l;
.P
    l = (rwlock *) arg;
    if ((--l->waiting_writers == 0) && (l->lock_count >= 0)) {
       /*
        * This only happens if we have been canceled. If the
        * lock is not held by a writer, there may be readers who
        * were blocked because waiting_writers was positive; they
        * can now be unblocked.
        */
        pthread_cond_broadcast(&l->rcond);
    }
    pthread_mutex_unlock(&l->lock);
}
.P
void
lock_for_write(rwlock *l)
{
    pthread_mutex_lock(&l->lock);
    l->waiting_writers++;
    pthread_cleanup_push(waiting_writer_cleanup, l);
    while (l->lock_count != 0)
        pthread_cond_wait(&l->wcond, &l->lock);
    l->lock_count = -1;
   /*
    * Note the pthread_cleanup_pop executes
    * waiting_writer_cleanup.
    */
    pthread_cleanup_pop(1);
}
.P
void
release_write_lock(rwlock *l)
{
    pthread_mutex_lock(&l->lock);
    l->lock_count = 0;
    if (l->waiting_writers == 0)
        pthread_cond_broadcast(&l->rcond);
    else
        pthread_cond_signal(&l->wcond);
    pthread_mutex_unlock(&l->lock);
}
.P
/*
 * This function is called to initialize the read/write lock.
 */
void
initialize_rwlock(rwlock *l)
{
    pthread_mutex_init(&l->lock, pthread_mutexattr_default);
    pthread_cond_init(&l->wcond, pthread_condattr_default);
    pthread_cond_init(&l->rcond, pthread_condattr_default);
    l->lock_count = 0;
    l->waiting_writers = 0;
}
.P
reader_thread()
{
    lock_for_read(&lock);
    pthread_cleanup_push(release_read_lock, &lock);
   /*
    * Thread has read lock.
    */
    pthread_cleanup_pop(1);
}
.P
writer_thread()
{
    lock_for_write(&lock);
    pthread_cleanup_push(release_write_lock, &lock);
   /*
    * Thread has write lock.
    */
pthread_cleanup_pop(1);
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
The two routines that push and pop cancellation cleanup handlers,
\fIpthread_cleanup_push\fR()
and
\fIpthread_cleanup_pop\fR(),
can be thought of as left and right-parentheses. They always need to
be matched.
.SH RATIONALE
The restriction that the two routines that push and pop
cancellation cleanup handlers,
\fIpthread_cleanup_push\fR()
and
\fIpthread_cleanup_pop\fR(),
have to appear in the same lexical scope allows for efficient macro or
compiler implementations and efficient storage management. A sample
implementation of these routines as macros might look like this:
.sp
.RS 4
.nf

#define pthread_cleanup_push(rtn,arg) { \e
    struct _pthread_handler_rec __cleanup_handler, **__head; \e
    __cleanup_handler.rtn = rtn; \e
    __cleanup_handler.arg = arg; \e
    (void) pthread_getspecific(_pthread_handler_key, &__head); \e
    __cleanup_handler.next = *__head; \e
    *__head = &__cleanup_handler;
.P
#define pthread_cleanup_pop(ex) \e
    *__head = __cleanup_handler.next; \e
    if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \e
}
.fi
.P
.RE
.P
A more ambitious implementation of these routines might do even better
by allowing the compiler to note that the
cancellation cleanup handler is a constant and can be expanded inline.
.P
This volume of POSIX.1\(hy2017 currently leaves unspecified the effect of calling
\fIlongjmp\fR()
from a signal handler executing in a POSIX System Interfaces function.
If an implementation wants to allow this and give the programmer
reasonable behavior, the
\fIlongjmp\fR()
function has to call all cancellation cleanup handlers that have been
pushed but not popped since the time
\fIsetjmp\fR()
was called.
.P
Consider a multi-threaded function called by a thread that uses
signals. If a signal were delivered to a signal handler during the
operation of
\fIqsort\fR()
and that handler were to call
\fIlongjmp\fR()
(which, in turn, did
.IR not
call the cancellation cleanup handlers) the helper threads created by
the
\fIqsort\fR()
function would not be canceled. Instead, they would continue to execute
and write into the argument array even though the array might have been
popped off the stack.
.P
Note that the specified cleanup handling mechanism is especially tied
to the C language and, while the requirement for a uniform mechanism
for expressing cleanup is language-independent, the mechanism used in
other languages may be quite different. In addition, this mechanism is
really only necessary due to the lack of a real exception mechanism in
the C language, which would be the ideal solution.
.P
There is no notion of a cancellation cleanup-safe function. If an
application has no cancellation points in its signal handlers, blocks
any signal whose handler may have cancellation points while calling
async-unsafe functions, or disables cancellation while calling
async-unsafe functions, all functions may be safely called from
cancellation cleanup routines.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIpthread_cancel\fR\^(\|)",
.IR "\fIpthread_setcancelstate\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<pthread.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
